The need and demand for inspection of metallic plates in an aging infrastructure has been increasing within the last decade due to an increase in both public awareness and concern for the environment. The non-destructive evaluation ("NDE") techniques currently in use do not meet all the requirements for such inspections. Recent market research indicates that there is a call for better inspection techniques. The technique and apparatus described herein provide for improved inspections of flat-shaped conducting structures.
Inspection of Above-Ground Storage Tanks ("ASTs") is one exemplary use for the present invention. Newspaper headlines have reported about AST failures and leakage plumes from some older structures. ASTs have the possible hazard of (and have actually caused) contaminating underground water supplies or generating run-off contamination into streams and rivers used as water supplies. The brittle fracture of a large AST near Pittsburgh, Pa., in January 1988, serves as a prime example of such catastrophes. About one million gallons (see Moore, Patrick O., "Report on Oil Spill Affirms Importance of Material Identification NDT", Materials Evaluation, Vol. 46, August 1988, P. 1128.) of petroleum products were discharged into the Monongahela River. A 1993 report from Environmental Defense Fund (see EDF Report, "Last But Not Least: Leaking Aboveground Storage Tank Threats, Costs, and Answers", Environment Defense Fund, Washington, D.C., Mar. 16, 1993.) indicated that a bulk fuel tank farm in spark, Nev., had 4-40 millon gallons of petroleum plumes and that there had been petroleum releases estimated to be from 80-252 millon gallons from a single California refinery.
There are approximately 300,000 (see EDF Report, "Last But Not Least: Leaking Aboveground Storage Tank Threats, Costs, and Answer", Environment defense Fund, Washington, D.C., Mar. 16. 1993.) to 1.2 millon (see Cater, Will, "Keynote and Overview of API653 Conference Proceedings", at API653 Conference, Jun. 10-11, 1993, sponsored by CEEM Information Services, Fairfax, Va.) tanks within the United States. This also includes tanks in chemical and other industries. Seventy percent of these tanks are twenty or more years old. Assuming each tank is inspected only once every twenty years, there is a need for inspecting 15,000-60,000 tanks per year in United States alone!
There are three NDE techniques that are popular and used for tank inspections:
(1) Acoatstic Emission ("AE"). PA0 (2) Ultrasonic Technique ("UT"). PA0 (3) Magnetic-Flux-Leakage ("MFL") Method.
AE work well for leak detection, but does anaiot tell the tank wall thickness, which is important for advanced awareness of possible impending leaks and for the prevention of any such leaks before they occur.
UT measures wall thickness accurately; however, manual UT is too slow for large area 100% coverage scanning. Automatic UT is able to inspect up to 500 ft.sup.2 /h, but it does not work well with bad surface conditions in the area to be scanned. Also, there are problems of using couplant (the material used to couple the ultrasonic energy from the probe to the plate under test) in the working site.
This method provides a high scan speed up to 1500 ft.sup.2 /h, so it is currently the most popular technique used for tank bottom inspection. However, as this method is sensitive to lift-off and has different sensitivity to near-side and far-side defects, it is used only as a tool for qualitative flow detection.
One remote field eddy current ("RFEC") technique was invented in 1951 (see MacClean, W. R., U.S. Pat. N0. 2,573,799, Nov. 1951, and Schmidt, T. R., "The Remote Field Eddy Current Inspection Technique.", Materials Evaluation, 42, pp. 225-230, February 1984) and is widely used as a nondestructive evaluation tool for inspecting metallic pipes and tubing. Essentially, the RFEC phenomenon can be observed when a coil is AC excited inside a conducting tube (see RFEC system 90 in prior-art Figure 1, in which excitation coil 91 is driven with an AC signal and creates direct signal path 95 and indirect signal path 96 which are detected by pick-up coil 94; defects in tubing 99 having center line 92 create signal changes as coil 91 and 94 are moved in tandem down the tubing 99). The RFEC signal can be sensed by a pick-up coil located 2-3 diameters away (i.e., 2 to 3 times the inner diameter dimension of the tubing) from the excitation coil. FIG. 1 shows a schematic of a prior-art RFEC probe system 90 for tube inspection and two signal paths 95 and 96, between the excitation coil 91 and the pick-up coil 94. The pick-up signal is closely related to the tube wall condition, thickness, permeability, and conductivity. The signal phase, especially, has approximately linear relationship with the tube wall thickness.
For tubing inspection, the RFEC technique is characterized by its substantially equal sensitivity to either an inner diameter ("ID") or an outer diameter ("OD") defect, its insensitivity to probe wobble or lift-off, and not being limited by the penetration depth, which has traditionally been a major disadvantage for conventional eddy-current techniques, especially in ferromagnetic material inspection. However, RFEC applications have been restricted to inspection of metallic tubing, although there is a need and demand for accurate and fast inspection for many flat-shaped metals, such as tank bottoms and vessel walls.
FIG. 2 shows the basic characteristics of the RFEC effect in a tubular product. Shown are two curves representing the logarithm of signal magnitude and the signal phase angle, respectively, as functions of the distance between the excitation and the pick-up coils. There are apparently two distinct regions, a near-field region and a remote-field region, separated by a transition zone. In the near field region, the signal magnitude attenuates exponentially, while the phase keeps approximately a constant value close to -90.degree.. In the remote-field region, the magnitude attenuation rate is significantly reduced, while the phase keeps a constant value, but on which is different from that in the near-field region. The phase difference is approximately proportional to twice the wall thickness. In the region between the two regions, the transition zone, there is a rapid change in the magnitude attenuation rate and the phase value.
Users desire probes and techniques that are fast, reliable, accurate, easy to operate, and not expensive. One purpose of the present invention is to extend the RFEC technique to planar metallic plates. The proposed plate remote field eddy current ("PRFEC") technique herein meets all of the above requirements.
There is also a need therefore for an analysis of the PRFEC effect to be observed on metallic plates, and for a PRFEC probe for inspection of objects with flat geometry, or objects with approximately flat geometry in a least a local area.